Polymer composition and a power cable comprising the polymer composition

ABSTRACT

An alternating current (AC) power cable includes a conductor surrounded by at least an inner semiconductive layer including a first semiconductive composition, an insulation layer including a polymer composition, an outer semiconductive layer including a second semiconductive composition, and optionally a jacketing layer including a jacketing composition, in that order. The polymer composition of the insulation layer includes an unsaturated low density polyethylene (LDPE) copolymer of ethylene with one or more polyunsaturated comonomers and a crosslinking agent. The polymer composition of the insulation layer has a dielectric loss expressed as tan δ (50 Hz) of 12.0×10−4 or less, when measured at 25 kV/mm and 130° C. according to “Test for Tan δ measurements on 10 kV cables”.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

BACKGROUND OF THE INVENTION Field of the Invention

The invention concerns a polymer composition suitable for a layer of apower cable, the use of the polymer composition in a layer of a powercable, a power cable comprising the polymer composition and to a processfor producing the cable.

Description of the Related Art

Polyolefins produced in a high pressure (HP) process are widely used indemanding polymer applications wherein the polymers must meet highmechanical and/or electrical requirements. For instance in power cableapplications, particularly in medium voltage (MV) and especially in highvoltage (HV) and extra high voltage (EHV) cable applications theelectrical properties of the polymer composition has a significantimportance. Furthermore, the requirement for the electrical propertiesmay differ in different cable applications, as is the case betweenalternating current (AC) and direct current (DC) cable applications.

A typical power cable comprises a conductor surrounded, at least, by aninner semiconductive layer, an insulation layer and an outersemiconductive layer, in that order.

Space Charge

There is a fundamental difference between AC and DC with respect toelectrical field distribution in the cable. The electric field in an ACcable is easily calculated since it depends on one material propertyonly, namely the relative permittivity (the dielectric constant) withknown temperature dependence. The electric field will not influence thedielectric constant. On the other hand, the electric field in a DC cableis much more complex and depends on the conduction, trapping andbuild-up of electric charges, so called space charges, inside theinsulation. Space charges inside the insulation will distort theelectric field and may lead to points of very high electric stress,possibly that high that a dielectric failure will follow.

Normally space charges are located close to the electrodes; charges ofthe same polarity as the nearby electrode are called homocharges,charges of opposite polarity are called heterocharges. The heterochargeswill increase the electric field at this electrode, homocharges willinstead reduce the electric field.

Tan δ (Dielectric Losses)

The tan δ and thus the dielectric losses (which are linearlyproportional to the tan δ) shall be as low as possible for bothtechnical and economical reasons:

-   -   Low losses means that low amount of transmitted electric energy        is lost as thermal energy inside the cable insulation. These        losses will mean economic losses for the power line operator.    -   Low losses will reduce the risk for thermal runaway, i.e. an        unstable situation where the temperature of the insulation will        increase due to the tan δ. When the temperature is increased,        normally the tan δ will also increase. This will further        increase the dielectric losses, and thus the temperature. The        results will be a dielectric failure of the cable that needs to        be replaced.

Compressor Lubricants

HP process is typically operated at high pressures up to 4000 bar. Inknown HP reactor systems the starting monomer(s) need to be compressed(pressurised) before introduced to the actual high pressurepolymerization reactor. Compressor lubricants are conventionally used inthe hyper-compressor(s) for cylinder lubrication to enable themechanically demanding compression step of starting monomer(s). It iswell known that small amounts of the lubricant normally leaks throughthe seals into the reactor and mixes with the monomer(s). In consequencethe reaction mixture contains traces (up to hundreds of ppm) of thecompressor lubricant during the actual polymerization step of themonomer(s). These traces of compressor lubricants can have an effect onthe electrical properties of the final polymer.

As examples of commercial compressor lubricants e.g. polyalkylene glycol(PAG): R—[C_(x)R_(y)H_(z)—O]_(n)—H, wherein R can be H or straight chainor branched hydrocarbyl and x, y, x, n are independent integers that canvary in a known manner, and lubricants based on a mineral oil(by-product in the distillation of petroleum) can be mentioned.Compressor lubricants which are based on mineral oils that meet therequirements set for the white mineral oil in European Directive2002/72/EC, Annex V, for plastics used in food contact, are used e.g.for polymerizing polymers especially for the food and pharmaceuticalindustry. Such mineral oil-based lubricants contain usually lubricityadditive(s) and may also contain other type of additive(s), such asantioxidants.

WO2009012041 of Dow discloses that in high pressure polymerizationprocess, wherein compressors are used for pressurizing the reactants,i.e. one or more monomer(s), the compressor lubricant may have an effecton the properties of the polymerized polymer. The document describes theuse of a polyol polyether which comprises one or none hydroxylfunctionality as a compressor lubricant for preventing prematurecrosslinking particularly of silane-modified HP polyolefins.WO2009012092 of Dow discloses a composition which comprises a HP (i)polyolefin free of silane functionality and (ii) a hydrophobic polyetherpolyol of PAG type wherein at least 50% of its molecules comprise nomore than a single hydroxyl functionality. The component (ii) appears tooriginate from a compressor lubricant. The composition is i.a. for W&Capplications and is stated to reduce dielectrical losses in MV and HVpower cables, see page 2, paragraph 0006. In both applications it isstated that hydrophilic groups (e.g. hydroxyl groups) present in thecompressor lubricant can result in increased water uptake by the polymerwhich in turn can increase electrical losses or, respectively,pre-mature scorch, when the polymer is used as a cable layer material.The problems are solved by a specific PAG type of lubricant with reducedamount of hydroxyl functionalities.

There is a continuous need in the polymer field to find polymers whichare suitable for demanding polymer applications such as wire and cableapplications with high requirements and stringent regulations.

SUMMARY OF THE INVENTION Objects of the Invention

One of the objects of the present invention is to provide a polymercomposition for use in an insulating layer of an alternating (AC) powercable with improved properties, as well as to an alternating (AC) powercable with improved properties.

The invention and further objects thereof are described and defined indetails below.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to a use of a polymer composition comprising apolyolefin and a crosslinking agent and wherein the polymer compositionhas a dielectric loss expressed as tan δ (50 Hz) of 12.0×10⁻⁴ or less,when measured at 25 kV/mm and 130° C. according to “Test for Tan δmeasurements on 10 kV cables” as described in the description part under“Determination methods”, for producing an insulation layer of a MV, HVor EHV AC power cable, preferably of a HV or EHV AC power cable,comprising a first semiconductive composition, an insulation layercomprising a polymer composition, an outer semiconductive layercomprising a second semiconductive composition and optionally ajacketing layer comprising a jacketing composition, in that order.

Moreover, the invention is directed to an alternating current (AC) powercable, comprising a conductor surrounded by at least an innersemiconductive layer comprising a first semiconductive composition, aninsulation layer comprising a polymer composition, an outersemiconductive layer comprising a second semiconductive composition andoptionally a jacketing layer comprising a jacketing composition, in thatorder, wherein the polymer composition of the insulation layer comprisesa polyolefin and a crosslinking agent, and wherein the polymercomposition of the insulation layer has a dielectric loss expressed astan δ (50 Hz) of 12.0×10⁻⁴ or less, when measured at 25 kV/mm and 130°C. according to “Test for Tan δ measurements on 10 kV cables” asdescribed in the description part under “Determination methods”.

The cable of the invention is also referred herein shortly as “cable”.The polymer composition of the insulation layer of the cable is alsoreferred herein shortly as “Polymer composition” or “polymercomposition”. The term “tan δ” or “tan delta”, as used herein, meanstangent delta which is a well known measure of dielectric loss. Asmentioned the method for determining tan delta is described below under“Determination methods”.

The term “conductor” means herein above and below that the conductorcomprises one or more wires. Moreover, the cable may comprise one ormore such conductors. Preferably the conductor is an electricalconductor and comprises one or more metal wires.

The polymer composition of the insulation layer has surprisingly reduceddielectric losses expressed as tan delta at high temperature and highstress. Dielectric losses are due to both oscillation of dipoles (suchas carbonyls) and conduction of free charge carriers (electrons, ions).The relative importance of these mechanisms depends on parameters suchas temperature, electric field and frequency during the measurement. Atroom temperature and 50 Hz, the main contributor is clearly the dipoles.However, at temperatures above the melting point, especially under highelectric field, the contribution from the free charge carriers hasincreased significantly.

The polymer composition with unexpectedly low dielectric losses at highstress and high temperatures has thus advantageously low conductivityand is highly suitable layer material in insulation layers of powercables, preferably of alternating (AC) power cables. Moreover theunexpectedly low dielectric losses are maintained even when the polymercomposition is surrounded by the semiconductive layers.

The cable comprises preferably the jacketing layer. When thesemiconductive layers and the insulation layer are combined with theoptional, and preferable, jacketing layer, then superior AC power cableis obtained which is particularly suitable for use as a medium voltage(MV), high voltage (HV) or extra high voltage (EHV) AC power cable, morepreferably as an AC power cable operating at any voltages, preferably athigher than 36 kV, most preferably as a HV or EHV AC power cable.

The polyolefin of the polymer composition is preferably produced in ahigh pressure (HP) process. As well known, the high pressure reactorsystem typically comprises a compression zone for a) compressing one ormore starting monomer(s) in one or more compressor(s) which are alsoknown as hyper-compressor(s), a polymerization zone for b) polymerizingthe monomer(s) in one or more polymerization reactor(s) and a recoveryzone for c) separating unreacted products in one or more separators andfor recovering the separated polymer. Moreover, the recovery zone of theHP reactor system typically comprises a mixing and pelletizing section,such as pelletizing extruder, after the separator(s), for recovering theseparated polymer in form of pellets. The process is described in moredetails below.

Surprisingly, when a mineral oil is used in compressors for cylinderlubrication in a HP reactor system for compressing the startingmonomer(s), then the resulting polyolefin has surprisingly lowdielectric losses at high stress and high temperatures which contributeto the excellent electrical properties of the polymer composition in aninsulation layer of the cable as stated above or below.

Compressor lubricant means herein a lubricant used in compressor(s),i.e. in hypercompressor(s), for cylinder lubrication.

More preferably the polymer composition of the insulation layercomprises a polyolefin and a crosslinking agent, and the polyolefin isobtainable by a high pressure process comprising

(a) compressing one or more monomer(s) under pressure in a compressor,using a compressor lubricant for lubrication,

(b) polymerizing a monomer optionally together with one or morecomonomer(s) in a polymerization zone,

(c) separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone,

wherein in step a) the compressor lubricant comprises a mineral oil.

The resulting polymer composition has the above mentioned advantageousreduced dielectric losses at high temperatures and high stress.

The expressions “obtainable by the process” or “produced by the process”are used herein interchangeably and mean the category “product byprocess”, i.e. that the product has a technical feature which is due tothe preparation process.

Accordingly it is more preferable that the insulation layer of the cablecomprises a polymer composition which has a dielectric loss expressed astan δ (50 Hz) of 12.0×10⁻⁴ or less, when measured at 25 kV/mm and 130°C. according to “Test for Tan δ measurements on 10 kV cables” asdescribed below under “Determination methods”; and

wherein the polyolefin of the polymer composition is obtainable by ahigh pressure process comprising

(a) compressing one or more monomer(s) under pressure in a compressor,using a compressor lubricant for lubrication,

(b) polymerizing a monomer optionally together with one or morecomonomer(s) in a polymerization zone,

(c) separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone,

wherein in step a) the compressor lubricant comprises a mineral oil.

Even more preferably the polymer composition of the insulation layer hasa dielectric loss expressed as tan δ (50 Hz) of 12.0×10⁻⁴ or less,preferably of 11.0×10⁻⁴ or less, preferably of 0.01-10.0×10⁻⁴, morepreferably of 0.1-9.0×10⁻⁴, more preferably of 0.3-8.0×10⁻⁴, morepreferably of 0.5-7.0×10⁻⁴, when measured at 25 kV/mm and 130° C.according to “Test for Tan δ measurements on 10 kV cables” as describedbelow under “Determination methods”.

In a more preferable embodiment of the cable, at least the polymercomposition of the insulation layer is crosslinkable and is crosslinkedin the presence of the crosslinking agent before the end use applicationof the cable.

“Crosslinkable” means that the polymer composition can be crosslinkedusing a crosslinking agent(s) before the use in the end applicationthereof. Crosslinkable polymer composition comprises the polyolefin andthe crosslinking agent. It is preferred that the polyolefin of thepolymer composition is crosslinked. The crosslinking of the polymercomposition is carried out at least with the crosslinking agent of thepolymer composition of the invention. Moreover, the crosslinked polymercomposition or, respectively, the crosslinked polyolefin, is mostpreferably crosslinked via radical reaction with a free radicalgenerating agent. The crosslinked polymer composition has a typicalnetwork, i.a. interpolymer crosslinks (bridges), as well known in thefield. As evident for a skilled person, the crosslinked polymer can beand is defined herein with features that are present in the polymercomposition or polyolefin before or after the crosslinking, as stated orevident from the context. For instance the presence of the crosslinkingagent in the polymer composition or the type and compositional property,such as MFR, density and/or unsaturation degree, of the polyolefincomponent are defined, unless otherwise stated, before crosslinking, andthe features after the crosslinking are e.g. the electrical property orcrosslinking degree measured from the crosslinked polymer composition.

The preferred crosslinking agent of the polymer composition is a freeradical generating agent(s), more preferably a peroxide(s).

Accordingly, the present preferable crosslinked polymer composition isobtainable by crosslinking with peroxide as defined above or below. Theexpressions “obtainable by crosslinking”, “crosslinked with” and“crosslinked polymer composition” are used herein interchangeably andmean that the crosslinking step provides a further technical feature tothe polymer composition as will be explained below.

It is evident to a skilled person that the cable can optionally compriseone or more other layer(s) comprising one or more screen(s), a jacketinglayer(s) or other protective layer(s), which layer(s) are conventionallyused in of W&C field.

The below preferable subgroups, properties and embodiments of thepolymer composition, the first semiconductive composition, secondsemiconductive composition and jacketing composition prior or after anyoptional crosslinking apply equally and independently to thecompositions and layers as such, as well as to the crosslinkable cableand the crosslinked cable, as defined above or below.

Preferably, the polymer composition comprises the crosslinking agent,preferably peroxide, in an amount of less than 10 wt %, less than 6 wt%, more preferably of less than 5 wt %, less than 3.5 wt %, even morepreferably from 0.1 wt % to 3 wt %, and most preferably from 0.2 wt % to2.6 wt %, based on the total weight of the polymer composition.

Peroxide is the preferred crosslinking agent. Non-limiting examples areorganic peroxides, such as di-tert-amylperoxide,2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne,2,5-di(tert-butylperoxy)-2,5-dimethylhexane, tert-butylcumylperoxide,di(tert-butyl)peroxide, dicumylperoxide,butyl-4,4-di(tert-butylperoxy)-valerate,1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane,tert-butylperoxybenzoate, dibenzoylperoxide, di(tertbutylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane,1,1-di(tert-butylperoxy)cyclohexane, 1,1-di(tert amylperoxy)cyclohexane,or any mixtures thereof. Preferably, the peroxide is selected from2,5-di(tert-butylperoxy)-2,5-dimethylhexane,di(tert-butylperoxyisopropyl)benzene, dicumylperoxide,tert-butylcumylperoxide, di(tert-butyl)peroxide, or mixtures thereof.Most preferably, the peroxide is dicumylperoxide.

In addition to crosslinking agent(s) the polymer composition with theadvantageous electrical properties may comprise further component(s),such as further polymer component(s) and/or one or more additive(s). Asoptional additives the polymer composition may contain antioxidant(s),stabiliser(s), water tree retardant additive(s), processing aid(s),scorch retarder(s), metal deactivator(s), crosslinking booster(s), flameretardant additive(s), acid or ion scavenger(s), inorganic filler(s),voltage stabilizer(s) or any mixtures thereof.

In a more preferable embodiment the polymer composition comprises one ormore antioxidant(s) and optionally one or more scorch retarder(s) (SR).

As non-limiting examples of antioxidants e.g. sterically hindered orsemi-hindered phenols, aromatic amines, aliphatic sterically hinderedamines, organic phosphites or phosphonites, thio compounds, and mixturesthereof, can be mentioned. As non-limiting examples of thio compounds,for instance

1. sulphur containing phenolic antioxidant(s), preferably selected fromthiobisphenol(s), the most preferred being 4,4′-thiobis(2-tertbutyl-5-methylphenol) (CAS number: 96-69-5), 2,2′-thiobis(6-t-butyl-4-methylphenol), 4,4′-thiobis (2-methyl-6-t-butylphenol),thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, or4,6-bis (octylthiomethyl)-o-cresol (CAS: 110553-27-0) or derivativesthereof; or any mixtures thereof,

2. Other thio compounds like di-stearyl-thio-dipropionate or similarcompounds with various length on the carbon chains; or mixtures thereof,

3. or any mixtures of 1) and 2).

Group 1) above is the preferred antioxidant(s).

In this preferable embodiment the amount of an antioxidant is preferablyfrom 0.005 to 2.5 wt-% based on the weight of the Polymer composition.The antioxidant(s) are preferably added in an amount of 0.005 to 2.0wt-%, more preferably 0.01 to 1.5 wt-%, even more preferably 0.03 to 0.8wt-%, even more preferably 0.04 to 0.8 wt-%based on the weight of thepolymer composition.

In a further preferable embodiment the polymer composition comprises atleast one or more antioxidant(s) and one or more scorch retarder(s).

The scorch retarder (SR) is well known additive type in the field andcan i.a. prevent premature crosslinking. As also known the SRs may alsocontribute to the unsaturation level of the polymer composition. Asexamples of scorch retarders, allyl compounds, such as dimers ofaromatic alpha-methyl alkenyl monomers, preferably2,4-di-phenyl-4-methyl-1-pentene, substituted or unsubstituteddiphenylethylenes, quinone derivatives, hydroquinone derivatives,monofunctional vinyl containing esters and ethers, monocyclichydrocarbons having at least two or more double bonds, or mixturesthereof, can be mentioned. Preferably, the amount of a scorch retarderis within the range of 0.005 to 2.0 wt.-%, more preferably within therange of 0.005 to 1.5 wt.-%, based on the weight of the Polymercomposition. Further preferred ranges are e.g. from 0.01 to 0.8 wt %,0.02 to 0.75 wt %, 0.02 to 0.70 wt %, or 0.03 to 0.60 wt %, based on theweight of the polymer composition. Preferred SR of the polymercomposition is 2,4-Diphenyl-4-methyl-1-pentene (CAS number 6362-80-7).

The polymer composition of the invention comprises typically at least 50wt %, preferably at least 60 wt %, more preferably at least 70 wt %,more preferably at least 75 wt %, more preferably from 80 to 100 wt %and more preferably from 85 to 100 wt %, of the polyolefin based on thetotal weight of the polymer component(s) present in the polymercomposition. The preferred polymer composition consists of polyolefin asthe only polymer component. The expression means that the polymercomposition does not contain further polymer components, but thepolyolefin as the sole polymer component. However, it is to beunderstood herein that the polymer composition may comprise furthercomponent(s) other than polymer components, such as additive(s) whichmay optionally be added in a mixture with a carrier polymer, i.e. in socalled master batch.

In an even more preferable embodiment of the cable, at least the firstsemiconductive composition of the inner semiconductive layer and thepolymer composition of the insulation layer are crosslinked before theend use application of the cable. Also the jacketing composition of theoptional, and preferable, jacketing layer may be crosslinked.

Moreover, each of the first and second semiconductive compositions andthe optional, and preferable, jacketing composition, when crosslinked,may comprise any crosslinking agent and is preferably crosslinked in aconventional manner using conventional amounts of the used crosslinkingagent. For instance any of the semiconductive compositions or theoptional, and preferable, jacketing composition can be crosslinkable bya peroxide or via crosslinkable groups, such as via hydrolysable silanegroups. Peroxide is preferably used in the above given amounts. Thehydrolysable silane groups may be introduced into the polymer of thecomposition by copolymerization of olefin, preferably ethylene,monomer(s) with silane group containing comonomers or by grafting thepolymer with silane groups containing compounds, i.e. by chemicalmodification of the polymer by addition of silane groups mostly in aradical reaction. Such silane groups containing comonomers and compoundsare well known in the field and e.g. commercially available. Thehydrolysable silane groups are typically then crosslinked by hydrolysisand subsequent condensation in the presence of a silanol-condensationcatalyst and H₂O in a manner known in the art. Also silane crosslinkingtechnique is well known in the art.

The invention is directed also to a process for producing acrosslinkable and crosslinked alternating current (AC) power cable, asdefined above or below, using the polymer composition of the invention.

Polyolefin Component of the Polymer Composition of the Insulation Layerof the Cable

The following preferable embodiments, properties and subgroups of thepolyolefin component suitable for the polymer composition aregeneralisable so that they can be used in any order or combination tofurther define the preferable embodiments of the polymer composition.Moreover, it is evident that the given description applies to thepolyolefin before it is crosslinked.

The term polyolefin means both an olefin homopolymer and a copolymer ofan olefin with one or more comonomer(s). As well known “comonomer”refers to copolymerizable comonomer units.

The polyolefin can be any polyolefin, such as a conventional polyolefin,which is suitable as a polymer in an insulating layer, of the AC powercable.

The polyolefin can be e.g. a commercially available polymer or can beprepared according to or analogously to known polymerization processdescribed in the chemical literature. More preferably the polyolefin isa polyethylene produced in a high pressure process, more preferably alow density polyethylene LDPE produced in a high pressure process. Themeaning of LDPE polymer is well known and documented in the literature.Although the term LDPE is an abbreviation for low density polyethylene,the term is understood not to limit the density range, but covers theLDPE-like HP polyethylenes with low, medium and higher densities. Theterm LDPE describes and distinguishes only the nature of HP polyethylenewith typical features, such as different branching architecture,compared to the PE produced in the presence of an olefin polymerizationcatalyst.

The LDPE as said polyolefin mean a low density homopolymer of ethylene(referred herein as LDPE homopolymer) or a low density copolymer ofethylene with one or more comonomer(s) (referred herein as LDPEcopolymer). The one or more comonomers of LDPE copolymer are preferablyselected from the polar comonomer(s), non-polar comonomer(s) or from amixture of the polar comonomer(s) and non-polar comonomer(s), as definedabove or below. Moreover, said LDPE homopolymer or LDPE copolymer assaid polyolefin may optionally be unsaturated.

As a polar comonomer for the LDPE copolymer as said polyolefin,comonomer(s) containing hydroxyl group(s), alkoxy group(s), carbonylgroup(s), carboxyl group(s), ether group(s) or ester group(s), or amixture thereof, can be used. More preferably, comonomer(s) containingcarboxyl and/or ester group(s) are used as said polar comonomer. Stillmore preferably, the polar comonomer(s) of LDPE copolymer is selectedfrom the groups of acrylate(s), methacrylate(s) or acetate(s), or anymixtures thereof. If present in said LDPE copolymer, the polarcomonomer(s) is preferably selected from the group of alkyl acrylates,alkyl methacrylates or vinyl acetate, or a mixture thereof. Furtherpreferably, said polar comonomers are selected from C₁- to C₆-alkylacrylates, C₁- to C₆-alkyl methacrylates or vinyl acetate. Still morepreferably, said polar LDPE copolymer is a copolymer of ethylene withC₁- to C₄-alkyl acrylate, such as methyl, ethyl, propyl or butylacrylate, or vinyl acetate, or any mixture thereof.

As the non-polar comonomer(s) for the LDPE copolymer as said polyolefin,comonomer(s) other than the above defined polar comonomers can be used.Preferably, the non-polar comonomers are other than comonomer(s)containing hydroxyl group(s), alkoxy group(s), carbonyl group(s),carboxyl group(s), ether group(s) or ester group(s). One group ofpreferable non-polar comonomer(s) comprise, preferably consist of,monounsaturated (=one double bond) comonomer(s), preferably olefins,preferably alpha-olefin(s), more preferably C₃ to C₁₀ alpha-olefin(s),such as propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, styrene,1-octene, 1-nonene; polyunsaturated (=more than one double bond)comonomer(s); a silane group containing comonomer(s); or any mixturesthereof. The polyunsaturated comonomer(s) are further described below inrelation to unsaturated LDPE copolymers.

If the LDPE polymer is a copolymer, it preferably comprises 0.001 to 50wt.-%, more preferably 0.05 to 40 wt.-%, still more preferably less than35 wt.-%, still more preferably less than 30 wt.-%, more preferably lessthan 25 wt.-%, of one or more comonomer(s). The Polymer composition,preferably the polyolefin component thereof, more preferably the LDPEpolymer, may optionally be unsaturated, i.e. the polymer composition,preferably the polyolefin, preferably the LDPE polymer, may comprisevinyl groups. The “unsaturated” means herein that the polymercomposition, preferably the polyolefin, contains vinyl groups/1000carbon atoms in a total amount of at least 0.04/1000 carbon atoms. Ingeneral, “vinyl group” means herein CH₂═CH— moiety.

As well known the unsaturation can be provided to the polymercomposition i.a. by means of the polyolefin, a low molecular weight (Mw)compound(s), such as crosslinking booster(s) or scorch retarderadditive(s), or any combinations thereof. If two or more above sourcesof vinyl groups are chosen to be used for providing the unsaturation,then the total amount of vinyl groups in the polymer composition meansthe sum of the vinyl groups present in the vinyl group sources. Thecontent of the vinyl groups is determined according to description partof the “Method for determination of the amount of double bonds in thepolymer composition or in a polymer” under the “Determination methods”which relates to measurement of the vinyl group content.

Any vinyl group measurements are carried out prior to crosslinking.

If the Polymer composition is unsaturated prior to crosslinking, then itis preferred that the unsaturation originates at least from anunsaturated polyolefin component. More preferably, the unsaturatedpolyolefin is an unsaturated polyethylene, more preferably anunsaturated LDPE polymer, even more preferably an unsaturated LDPEhomopolymer or an unsaturated LDPE copolymer. When polyunsaturatedcomonomer(s) are present in the LDPE polymer as said unsaturatedpolyolefin, then the LDPE polymer is an unsaturated LDPE copolymer.

If an LDPE homopolymer is unsaturated, then the unsaturation can beprovided e.g. by a chain transfer agent (CTA), such as propylene, and/orby polymerization conditions. If an LDPE copolymer is unsaturated, thenthe unsaturation can be provided by one or more of the following means:by a chain transfer agent (CTA), by one or more polyunsaturatedcomonomer(s) or by polymerization conditions. It is well known thatselected polymerization conditions such as peak temperatures andpressure, can have an influence on the unsaturation level. In case of anunsaturated LDPE copolymer, it is preferably an unsaturated LDPEcopolymer of ethylene with at least one polyunsaturated comonomer, andoptionally with other comonomer(s), such as polar comonomer(s) which ispreferably selected from acrylate or acetate comonomer(s).

The polyunsaturated comonomers suitable for the unsaturated polyolefinpreferably consist of a straight carbon chain with at least 8 carbonatoms and at least 4 carbons between the non-conjugated double bonds, ofwhich at least one is terminal, more preferably, said polyunsaturatedcomonomer is a diene, preferably a diene which comprises at least eightcarbon atoms, the first carbon-carbon double bond being terminal and thesecond carbon-carbon double bond being non-conjugated to the first one.Preferred dienes are selected from C₈ to C₁₄ non-conjugated dienes ormixtures thereof, more preferably selected from 1,7-octadiene,1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene,7-methyl-1,6-octadiene, 9-methyl-1,8-decadiene, or mixtures thereof.Even more preferably, the diene is selected from 1,7-octadiene,1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene, or any mixturethereof, however, without limiting to above dienes.

It is well known that e.g. propylene can be used as a comonomer or as achain transfer agent (CTA), or both, whereby it can contribute to thetotal amount of the C—C double bonds, preferably to the total amount ofthe vinyl groups. Herein, when a compound which can also act ascomonomer, such as propylene, is used as CTA for providing double bonds,then said copolymerizable comonomer is not calculated to the comonomercontent.

If the polyolefin, more preferably the LDPE polymer, is unsaturated,then it has preferably the total amount of vinyl groups is preferablyhigher than 0.05/1000 carbon atoms, still more preferably higher than0.08/1000 carbon atoms, still more preferably of higher than 0.11/1000carbon atoms and most preferably of higher than 0.15/1000 carbon atoms.Preferably, the total amount of vinyl groups is of lower than 4.0/1000carbon atoms. In some embodiments even higher unsaturation is desired,then the polyolefin, prior to crosslinking, contains preferably vinylgroups in total amount of more than 0.20/1000 carbon atoms, morepreferably more than 0.25/1000 carbon atoms, still more preferably ofmore than 0.30/1000 carbon atoms. The higher vinyl group amounts arepreferably provided by an unsaturated LDPE copolymer of ethylene with atleast one polyunsaturated comonomer.

The preferred polyolefin for use in the Polymer composition is asaturated LDPE homopolymer or a saturated LDPE copolymer of ethylenewith one or more comonomer(s) or an unsaturated LDPE polymer, which isselected from an unsaturated LDPE homopolymer or an unsaturated LDPEcopolymer of ethylene with one or more comonomer(s), preferably with atleast one polyunsaturated comonomer.

Typically, and preferably in W&C applications, the density of thepolyolefin, preferably of the LDPE polymer, is higher than 860 kg/m³.Preferably the density of the polyolefin, preferably of the LDPEpolymer, the ethylene homo- or copolymer is not higher than 960 kg/m³,and preferably is from 900 to 945 kg/m³. The MFR2 (2.16 kg, 190° C.) ofthe polyolefin, preferably of the LDPE polymer, is preferably from 0.01to 50 g/10 min, more preferably is from 0.1 to 20 g/10 min, and mostpreferably is from 0.2 to 10 g/10 min.

Compressor Lubricant

The compressor lubricant used in the polymerization process forproducing the preferred polyolefin of the Polymer composition comprisesmineral oil which is a known petroleum product.

Mineral oils have a well known meaning and are used i.a. for lubricationin commercial lubricants. “Compressor lubricant comprising a mineraloil” and “mineral oil-based compressor lubricants” are used hereininterchangeably.

Mineral oil can be a synthetic mineral oil which is producedsynthetically or a mineral oil obtainable from crude oil refineryprocesses.

Typically, mineral oil, known also as liquid petroleum, is a by-productin the distillation of petroleum to produce gasoline and other petroleumbased products from crude oil. The mineral oil of the compressorlubricant of the invention is preferably a paraffinic oil. Suchparaffinic oil is derived from petroleum based hydrocarbon feedstocks.

Mineral oil is preferably the base oil of the compressor lubricant. Thecompressor lubricant may comprise other components, such as lubricityadditive(s), viscosity builders, antioxidants, other additive(s) or anymixtures thereof, as well known in the art.

More preferably, the compressor lubricant comprises a mineral oil whichis conventionally used as compressor lubricants for producing plastics,e.g. LDPE, for food or medical industry, more preferably the compressorlubricant comprises a mineral oil which is a white oil. Even morepreferably the compressor lubricant comprises white oil as the mineraloil and is suitable for the production of polymers for food or medicalindustry. White oil has a well known meaning. Moreover such white oilbased compressor lubricants are well known and commercially available.Even more preferably the white oil meets the requirements for a food ormedical white oil.

As, known, the mineral oil, preferably the white mineral oil of thepreferred compressor lubricant contains paraffinic hydrocarbons.

Even more preferably, of the compressor lubricant meets one or more ofthe below embodiments:

-   -   In one preferable embodiment, the mineral oil, preferably the        white mineral oil, of the compressor lubricant has a viscosity        of at least 8.5×10⁻⁶ m²/s at 100° C.;    -   In a second preferable embodiment, the mineral oil, preferably        the white mineral oil, of the compressor lubricant contains 5%        per weight (wt %) or less of hydrocarbons with less than 25        carbon atoms;    -   In a third preferable embodiment, the hydrocarbons of the        mineral oil, preferably of the white mineral oil, of the        compressor lubricant have an average molecular weight (Mw) of        480 or more.

The above “amount of hydrocarbons”, “viscosity” and “Mw” are preferablyin accordance with the above European Directive 2002/72/EC of 6 Aug.2002.

It is preferred that the compressor lubricant is according to each ofthe above three embodiments 1-3.

The most preferred compressor lubricant of the invention meets therequirements given for white mineral oil in European Directive2002/72/EC of 6 Aug. 2002, Annex V, for plastics used in food contact.Directive is published e.g. in L 220/18 EN Official Journal of theEuropean Communities 15.8.2002. Accordingly the mineral oil is mostpreferably a white mineral oil which meets said European Directive2002/72/EC of 6 Aug. 2002, Annex V. Moreover it is preferred that thecompressor lubricant complies with said European Directive 2002/72/EC of6 Aug. 2002.

The compressor lubricant of the invention can be a commerciallyavailable compressor lubricant or can be produced by conventional means,and is preferably a commercial lubricant used in high pressurepolymerization processes for producing plastics for medical or foodapplications. Non-exhaustive examples of preferable commerciallyavailable compressor lubricants are e.g. Exxcolub R Series compressorlubricant for production of polyethylene used in food contact andsupplied i.a. by ExxonMobil, Shell Corena for producing polyethylene forpharmaceutical use and supplied by Shell, or CL-1000-SONO-EU, suppliedby Sonneborn.

The compressor lubricant contains preferably no polyalkyleneglycol basedcomponents.

It is preferred that any mineral oil present in the Polymer compositionof the invention originates from the compressor lubricant used in theprocess equipment during the polymerization process of the polyolefin.Accordingly, it is preferred that no mineral oil is added to the Polymercomposition or to the polyolefin after the polymerization thereof.

Traces of the mineral oil originating from the compressor lubricant andpresent, if any, in the produced polyolefin would typically amount inmaximum of up to 0.4 wt % based on the amount of the polyolefin. Thegiven limit is the absolute maximum based on the calculation of theworst scenario where all the lost compressor lubricant (average leakage)would go to the final polyolefin. Such worst scenario is unlikely andnormally the resulting polyolefin contains clearly lower level of themineral oil.

The compressor lubricant of the invention is used in a conventionalmanner and well known to a skilled person for the lubrication of thecompressor(s) in the compressing step (a) of the invention.

Process

The high pressure (HP) process is the preferred process for producing apolyolefin of the Polymer composition, preferably a low densitypolyethylene (LDPE) polymer selected from LDPE homopolymer or LDPEcopolymer of ethylene with one or more comonomers.

The invention further provides a process for polymerizing a polyolefinin a high pressure process which comprises the steps of:

(a) compressing one or more monomer(s) under pressure in a compressor,wherein a compressor lubricant is used for lubrication,

(b) polymerizing a monomer optionally together with one or morecomonomer(s) in a polymerization zone(s),

(c) separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone,

wherein in step a) a compressor lubricant comprises a mineral oilincluding the preferable embodiments thereof.

Accordingly, the polyolefin of the invention is preferably produced athigh pressure by free radical initiated polymerization (referred to ashigh pressure radical polymerization). The preferred polyolefin is LDPEhomopolymer or LDPE copolymer of ethylene with one or more comonomer(s),as defined above. The LDPE polymer obtainable by the process of theinvention preferably provides the advantageous electrical properties asdefined above or below. The high pressure (HP) polymerization and theadjustment of process conditions for further tailoring the otherproperties of the polyolefin depending on the desired end applicationare well known and described in the literature, and can readily be usedby a skilled person.

Compression Step a) of the Process of the Invention:

Monomer, preferably ethylene, with one or more optional comonomer(s), isfed to one or more compressor at compressor zone to compress themonomer(s) up to the desired polymerization pressure and to enablehandling of high amounts of monomer(s) at controlled temperature.Typical compressors, i.e. hyper-compressors, for the process can bepiston compressors or diaphragm compressors. The compressor zone usuallycomprises one or more compressor(s), i.e. hyper-compressor(s), which canwork in series or in parallel. The compressor lubricant of the inventionis used for cylinder lubrication in at least one, preferably in all ofthe hyper-compressor(s), present in the compressor zone. The compressionstep a) comprises usually 2-7 compression steps, often with intermediatecooling zones. Temperature is typically low, usually in the range ofless than 200° C., preferably of less than 100° C. Any recycled monomer,preferably ethylene, and optional comonomer(s) can be added at feasiblepoints depending on the pressure.

Polymerization Step b) of the Process:

Preferred high pressure polymerization is effected at a polymerizationzone which comprises one or more polymerization reactor(s), preferablyat least a tubular reactor or an autoclave reactor, preferably a tubularreactor. The polymerization reactor(s), preferably a tubular reactor,may comprise one or more reactor zones, wherein different polymerizationconditions may occur and/or adjusted as well known in the HP field. Oneor more reactor zone(s) are provided in a known manner with means forfeeding monomer and optional comonomer(s), as well as with means foradding initiator(s) and/or further components, such as CTA(s).Additionally, the polymerization zone may comprise a preheating sectionwhich is preceding or integrated to the polymerization reactor. In onepreferable HP process the monomer, preferably ethylene, optionallytogether with one or more comonomer(s) is polymerized in a preferabletubular reactor, preferably in the presence of chain transfer agent(s).

Tubular Reactor:

The reaction mixture is fed to the tubular reactor. The tubular reactormay be operated as a single-feed system (also known as front feed),wherein the total monomer flow from the compressor zone is fed to theinlet of the first reaction zone of the reactor. Alternatively thetubular reactor may be a multifeed system, wherein e.g the monomer(s),the optional comonomer(s) or further component(s) (like CTA(S)) comingfrom the compression zone, separately or in any combinations, is/aresplit to two or more streams and the split feed(s) is introduced to thetubular reactor to the different reaction zones along the reactor. Forinstance 10-90% of the total monomer quantity is fed to the firstreaction zone and the other 90-10% of the remaining monomer quantity isoptionally further split and each split is injected at differentlocations along the reactor. Also the feed of initiator(s) may be splitin two or more streams. Moreover, in a multifeed system the splitstreams of monomer(/comonomer) and/or optional further component(s),such as CTA, and, respectively, the split streams of initiator(s) mayhave the same or different component(s) or concentrations of thecomponents, or both.

The single feed system for the monomer and optional comonomer(s) ispreferred in the tubular reactor for producing the polyolefin of theinvention.

First part of the tubular reactor is to adjust the temperature of thefeed of monomer, preferably ethylene, and the optional comonomer(s);usual temperature is below 200° C., such as 100-200° C. Then the radicalinitiator is added. As the radical initiator, any compound or a mixturethereof that decomposes to radicals at an elevated temperature can beused. Usable radical initiators, such as peroxides, are commerciallyavailable. The polymerization reaction is exothermic. There can beseveral radical initiator injections points, e.g. 1-5 points, along thereactor usually provided with separate injection pumps. As alreadymentioned also the monomer, preferably ethylene, and optionalcomonomer(s), is added at front and optionally the monomer feed(s) canbe split for the addition of the monomer and/or optional comonomer(s),at any time of the process, at any zone of the tubular reactor and fromone or more injection point(s), e.g. 1-5 point(s), with or withoutseparate compressors.

Furthermore, one or more CTA(s) are preferably used in thepolymerization process of the Polyolefin. Preferred CTA(s) can beselected from one or more non-polar and one or more polar CTA(s), or anymixtures thereof.

Non-polar CTA, if present, is preferably selected from

i) one or more compound(s) which does not contain a polar group selectedfrom nitrile (CN), sulfide, hydroxyl, alkoxy, aldehyl (HC═O), carbonyl,carboxyl, ether or ester group(s), or mixtures thereof. Non-polar CTA ispreferably selected from one or more non-aromatic, straight chainbranched or cyclic hydrocarbyl(s), optionally containing a hetero atomsuch as O, N, S, Si or P. More preferably the non-polar CTA(s) isselected from one or more cyclic alpha-olefin(s) of 5 to 12 carbon orone or more straight or branched chain alpha-olefin(s) of 3 to 12 carbonatoms, more preferably from one or more straight or branched chainalpha-olefin(s) of 3 to 6 carbon atoms. The preferred non-polar CTA ispropylene.

The polar CTA, if present, is preferably selected from

i) one or more compound(s) comprising one or more polar group(s)selected from nitrile (CN), sulfide, hydroxyl, alkoxy, aldehyl (HC═O),carbonyl, carboxyl, ether or ester group(s), or mixtures thereof;

ii) one or more aromatic organic compound(s), or

iii) any mixture thereof.

Preferably any such polar CTA(s) have up to 12 carbon atoms, e.g. up to10 carbon atoms preferably up to 8 carbon atoms. A preferred optionincludes a straight chain or branched chain alkane(s) having up to 12carbon atoms (e.g. up to 8 carbon atoms) and having at least one nitrile(CN), sulfide, hydroxyl, alkoxy, aldehyl (HC═O), carbonyl, carboxyl orester group.

More preferably the polar CTA(s) , if present, is selected from i) oneor more compound(s) containing one or more hydroxyl, alkoxy, HC═O,carbonyl, carboxyl and ester group(s), or a mixture thereof, morepreferably from one or more alcohol, aldehyde and/or ketone compound(s).The preferred polar CTA(s) , if present, is a straight chain or branchedchain alcohol(s), aldehyde(s) or ketone(s) having up to 12 carbon atoms,preferably up to 8 carbon atoms, especially up to 6 carbon atoms, mostpreferably, isopropanol (IPA), methylethylketone (MEK) and/orpropionaldehyde (PA).

The amount of the preferable CTA(s) is not limited and can be tailoredby a skilled person within the limits of the invention depending on thedesired end properties of the final polymer. Accordingly, the preferablechain transfer agent(s) can be added in any injection point of thereactor to the polymer mixture. The addition of one or more CTA(s) canbe effected from one or more injection point(s) at any time during thepolymerization.

In case the polymerization of the polyolefin is carried out in thepresence of a CTA mixture comprising one or more polar CTA(s) as definedabove and one or more non-polar CTA(s) as defined above, then the feedratio by weight % of polar CTA to non-polar CTA is preferably

1 to 99 wt % of polar CTA and

1 to 99 wt % of non-polar CTA, based on the combined amount of the feedof polar CTA and the non-polar CTA into the reactor.

The addition of monomer, comonomer(s) and optional CTA(s) may includeand typically includes fresh and recycled feed(s).

The reactor is continuously cooled e.g. by water or steam. The highesttemperature is called peak temperature and the reaction startingtemperature is called initiation temperature.

Suitable temperatures range up to 400° C., preferably from 80 to 350° C.and pressure from 700 bar, preferably 1000 to 4000 bar, more preferablyfrom 1000 to 3500 bar. Pressure can be measured at least aftercompression stage and/or after the tubular reactor. Temperature can bemeasured at several points during all steps. High temperature and highpressure generally increase output. Using various temperature profilesselected by a person skilled in the art will allow control of structureof polymer chain, i.e. Long Chain Branching and/or Short Chainbranching, density, branching factor, distribution of comonomers, MFR,viscosity, Molecular Weight Distribution etc.

The reactor ends conventionally with a valve a so-called productioncontrol valve. The valve regulates reactor pressure and depressurizesthe reaction mixture from reaction pressure to separation pressure.

Recovering Step c) of the Process:

Separation:

The pressure is typically reduced to approx 100 to 450 bar and thereaction mixture is fed to a separator vessel where most of theunreacted, often gaseous, products are removed from the polymer stream.Unreacted products comprise e.g. monomer or the optional comonomer(s),and most of the unreacted components are recovered. The polymer streamis optionally further separated at lower pressure, typically less than 1bar, in a second separator vessel where more of the unreacted productsare recovered. Normally low molecular compounds, i.e. wax, are removedfrom the gas. The gas is usually cooled and cleaned before recycling.

Recovery of the Separated Polymer:

After the separation the obtained polymer is typically in a form of apolymer melt which is normally mixed and pelletized in a pelletizingsection, such as pelletizing extruder, arranged in connection to the HPreactor system. Optionally, additive(s), such as antioxidant(s), can beadded in this mixer in a known manner to result in the Polymercomposition.

Further details of the production of ethylene (co)polymers by highpressure radical polymerization can be found i.a. in the Encyclopedia ofPolymer Science and Engineering, Vol. 6 (1986), pp 383-410 andEncyclopedia of Materials: Science and Technology, 2001 Elsevier ScienceLtd.: “Polyethylene: High-pressure, R. Klimesch, D. Littmann and F.-O.Mähling pp. 7181-7184.

As to polymer properties, e.g. MFR, of the polymerized Polymer,preferably LDPE polymer, the properties can be adjusted by using e.g.chain transfer agent during the polymerization, or by adjusting reactiontemperature or pressure (which also to a certain extent have aninfluence on the unsaturation level).

When an unsaturated LDPE copolymer of ethylene is prepared, then, aswell known, the C—C double bond content can be adjusted by polymerizingthe ethylene e.g. in the presence of one or more polyunsaturatedcomonomer(s), chain transfer agent(s), process conditions, or anycombinations thereof, e.g. using the desired feed ratio between monomer,preferably ethylene, and polyunsaturated comonomer and/or chain transferagent, depending on the nature and amount of C—C double bonds desiredfor the unsaturated LDPE copolymer. I.a. WO 9308222 describes a highpressure radical polymerization of ethylene with polyunsaturatedmonomers, such as an α,ω-alkadienes, to increase the unsaturation of anethylene copolymer. The non-reacted double bond(s) thus provides i.a.pendant vinyl groups to the formed polymer chain at the site, where thepolyunsaturated comonomer was incorporated by polymerization. As aresult the unsaturation can be uniformly distributed along the polymerchain in random copolymerization manner. Also e.g. WO 9635732 describeshigh pressure radical polymerization of ethylene and a certain type ofpolyunsaturated α,ω-divinylsiloxanes. Moreover, as known, e.g. propylenecan be used as a chain transfer agent to provide said double bonds.

First and Second Semiconductive Compositions of the Cable

The first and second semiconductive compositions can be different oridentical and the following preferred embodiments of the semiconductivecomposition apply independently to each of them.

The semiconductive composition comprises preferably a polyolefin (S) anda conductive filler.

A suitable polyolefin (S) can be any polyolefin, such as anyconventional polyolefin, which can be used for producing asemiconductive cable layer of a cable, of the present invention. Forinstance such suitable conventional polyolefins are as such well knownand can be e.g. commercially available or can be prepared according toor analogously to known polymerization processes described in thechemical literature.

The polyolefin (S) for the polymer composition is preferably selectedfrom a polypropylene (PP) or polyethylene (PE), preferably from apolyethylene. For polyethylene, ethylene will form the major monomercontent present in any polyethylene polymer.

Preferable polyolefin (S) is a polyethylene produced in the presence ofan olefin polymerization catalyst or a polyethylene produced in a highpressure process.

In case a polyolefin (S) is a copolymer of ethylene with at least onecomonomer, then such comonomer(s) is selected from non-polarcomonomer(s) or polar comonomers, or any mixtures thereof. Preferableoptional non-polar comonomers and polar comonomers are described belowin relation to polyethylene produced in a high pressure process. Thesecomonomers can be used in any polyolefin (S) of the invention.

“Olefin polymerization catalyst” means herein preferably a conventionalcoordination catalyst. It is preferably selected from a Ziegler-Nattacatalyst, single site catalyst which term comprises a metallocene and anon-metallocene catalyst, or a chromium catalyst, or any mixturethereof. The terms have a well known meaning.

Polyethylene polymerized in the presence of an olefin polymerizationcatalyst is also often called as “low pressure polyethylene” todistinguish it clearly from polyethylene produced in a high pressure.Both expressions are well known in the polyolefin field. Low pressurepolyethylene can be produced in polymerization process operating i.a. inbulk, slurry, solution, or gas phase conditions or in any combinationsthereof. The olefin polymerization catalyst is typically a coordinationcatalyst as defined above.

More preferably, the polyolefin (S) is selected from a homopolymer or acopolymer of ethylene produced in the presence of a coordinationcatalyst or produced in a high pressure polymerization process.

Where the polyolefin (S) is a low pressure polyethylene (PE), then suchlow pressure PE is preferably selected from a very low density ethylenecopolymer (VLDPE), a linear low density ethylene copolymer (LLDPE), amedium density ethylene copolymer (MDPE) or a high density ethylenehomopolymer or copolymer (HDPE). These well known types are namedaccording to their density area. The term VLDPE includes hereinpolyethylenes which are also known as plastomers and elastomers andcovers the density range of from 850 to 909 kg/m³. The LLDPE has adensity of from 909 to 930 kg/m³, preferably of from 910 to 929 kg/m³,more preferably of from 915 to 929 kg/m³. The MDPE has a density of from930 to 945 kg/m³, preferably 931 to 945 kg/m³. The HDPE has a density ofmore than 945 kg/m³, preferably of more than 946 kg/m³, preferably from946 to 977 kg/m³, more preferably from 946 to 965 kg/m³.

More preferably, such low pressure copolymer of ethylene for thepolyolefin (S) is copolymerized with at least one comonomer selectedfrom C3-20 alpha olefin, more preferably from C4-12 alpha-olefin, morepreferably from C4-8 alpha-olefin, e.g. with 1-butene, 1-hexene or1-octene, or a mixture thereof. The amount of comonomer(s) present in aPE copolymer is from 0.1 to 15 mol %, typically 0.25 to 10 mol-%.

Moreover, where the polyolefin (S) is a low pressure PE polymer, thensuch PE can be unimodal or multimodal with respect to molecular weightdistribution (MWD=Mw/Mn). Generally, a polymer comprising at least twopolymer fractions, which have been produced under differentpolymerization conditions (including i.a. any of the process parameters,feeds of starting materials, feeds of process controlling agents andfeeds of catalyst systems) resulting in different (weight average)molecular weights and molecular weight distributions for the fractions,is referred to as “multimodal”. The prefix “multi” relates to the numberof different polymer fractions present in the polymer. Thus, forexample, multimodal polymer includes so called “bimodal” polymerconsisting of two fractions. The form of the molecular weightdistribution curve, i.e. the appearance of the graph of the polymerweight fraction as a function of its molecular weight, of a multimodalpolymer will show two or more maxima or is typically distinctlybroadened in comparison with the curves for the individual fractions.

Unimodal low pressure PE can be produced e.g. by a single stagepolymerization in a single reactor in a well known and documentedmanner. The multimodal (e.g. bimodal) low pressure PE can be producede.g. by blending mechanically together two or more separate polymercomponents or, preferably, by in-situ blending during the polymerizationprocess of the components. Both mechanical and in-situ blending are wellknown in the field. In-situ blending means the polymerization of thepolymer components under different polymerization conditions, e.g. in amultistage, i.e. two or more stage, polymerization process or by the useof two or more different polymerization catalysts, in a one stagepolymerization process, or by use a combination of multistagepolymerization process and two or more different polymerizationcatalysts. The polymerization zones may operate in bulk, slurry,solution, or gas phase conditions or in any combinations thereof, asknown in the field.

According to a second embodiment the polyolefin (S) is a polyethyleneproduced in a high pressure polymerization process, preferably byradical polymerization in the presence of an initiator(s). Morepreferably the polyolefin (S) is a low density polyethylene (LDPE). Whenthe polyolefin (S), preferably polyethylene, is produced in a highpressure process, then the preferred polyolefin is an LDPE homo polymeror an LDPE copolymer of ethylene with one or more comonomers. In someembodiments the LDPE homopolymer and copopolymer may be unsaturated.Examples of suitable LDPE polymers and general principles for theirpolymerization are described above in relation to polyolefin of thepolymer composition of the insulation layer, however, without limitingto any specific lubricant in the compressor(s) during the compressingstep (a) of the process. For the production of ethylene (co)polymers byhigh pressure radical polymerization, reference can be made to theEncyclopedia of Polymer Science and Engineering, Vol. 6 (1986), pp383-410 and Encyclopedia of Materials: Science and Technology, 2001Elsevier Science Ltd.: “Polyethylene: High-pressure, R. Klimesch, D.Littmann and F.-O. Mähling pp. 7181-7184.

The conductive filler of the semiconductive composition is preferably acarbon black. Any carbon black can be used which is electricallyconductive and provides semiconductive property needed for thesemiconductive layer.

Preferably, the carbon black may have a nitrogen surface area (BET) of 5to 400 m²/g, preferably of 10 to 300 m²/g, more preferably of 30 to 200m²/g, when determined according to ASTM D3037-93. Further preferably thecarbon black has one or more of the following properties: i) a primaryparticle size of at least 5 nm which is defined as the number averageparticle diameter according to ASTM D3849-95a procedure D, ii) iodineabsorption number (IAN) of at least 10 mg/g, preferably of 10 to 300mg/g, more preferably of 30 to 200 mg/g, when determined according toASTM D-1510-07; and/or iii) DBP (dibutyl phthalate) absorption number of60 to 300 cm³/100 g, preferably of 70 to 250 cm³/100 g, more preferablyof 80 to 200, preferably of 90 to 180 cm³/100 g, when measured accordingto ASTM D 2414-06a. More preferably the carbon black has a nitrogensurface area (BET) and properties (i), (ii) and (iii) as defined above.Non-limiting examples of preferable carbon blacks include furnace carbonblacks and acetylene blacks.

The amount of carbon black is at least such that a semiconductingcomposition is obtained. Depending on the desired use, the conductivityof the carbon black and conductivity of the composition, the amount ofcarbon black can vary.

Furnace carbon black is generally acknowledged term for the well knowncarbon black type that is produced in a furnace-type reactor. Asexamples of carbon blacks, the preparation process thereof and thereactors, reference can be made to e.g. EP629222 of Cabot, U.S. Pat.Nos. 4,391,789, 3,922,335 and 3,401,020. Furnace carbon black isdistinguished herein from acetylene carbon black which produced byreaction of acetylene and unsaturated hydrocarbons, e.g. as described inU.S. Pat. No. 4,340,577.

Acetylene black is a generally acknowledged term and are very well knownand e.g. supplied by Denka. They are produced in an acetylene blackprocess.

Preferably, the semiconductive composition of the cable has a volumeresistivity, measured at 90° C. according to ISO 3915 (1981), of lessthan 500,000 Ohm cm, more preferably less than 100,000 Ohm cm, even morepreferably less than 50,000 Ohm cm. Volume resistivity is in areciprocal relationship to electrical conductivity, i.e. the lower theresistivity, the higher is the conductivity.

The semiconductive composition of the present invention comprises,depending on the used carbon black, preferably 9.5 to 49.5 wt %, morepreferably 9 to 49 wt %, more preferably 5 to 45 wt % carbon black,based on the weight of the polymer composition.

The crosslinking option and usable crosslinking agents are describedabove in relation to the description of the polymer composition of theinsulation layer.

The semiconductive composition of the cable may naturally comprisefurther components, such as further polymer component(s), like misciblethermoplastic(s); or further additive(s), such as antioxidant(s), scorchretardant(s); additive(s), such as any of antioxidant(s), scorchretarder(s) (SR), water treeing retardant additive(s), crosslinkingbooster(s), stabiliser(s), like voltage stabilizer(s), flame retardantadditive(s), acid, ion scavenger(s), further filler(s), processingaid(s), like lubricant(s), foaming agent(s) or colorant(s), as known inthe polymer field. The additives depend on the type of the layer, e.g.whether semiconductive or insulation layer, and can be selected by askilled person. The total amount of further additive(s), if present, isgenerally from 0.01 to 10 wt %, preferably from 0.05 to 7 wt %, morepreferably from 0.2 to 5 wt %, based on the total amount of the polymercomposition.

The semiconductive composition of the cable of the invention comprisestypically at least 50 wt %, preferably at least 60 wt %, more preferablyat least 70 wt % to 100 wt %, of the polyolefin based on the totalweight of the polymer component(s) present in the semiconductivecomposition. However, it is to be understood herein that thesemiconductive composition may comprise further component(s) other thanpolymer components, such as additive(s) which may optionally be added ina mixture with a carrier polymer, i.e. in so called master batch.

Jacketing Composition of the Jacketing Layer of the Cable

The jacketing composition preferably comprises a polyolefin (j) which ispreferably selected from a polypropylene (PP), polyethylene (PE), or anymixtures thereof. More preferably, the polyolefin (j) of the jacketingcomposition is selected independently from the polyolefin (S) asdescribed for the first and second semiconductive composition.

The crosslinking option and usable crosslinking agents are describedabove in relation to the description of the polymer composition of theinsulation layer.

The jacketing composition may comprises further components such asfurther polymer component(s), further additive(s), such asantioxidant(s), stabiliser(s), fillers, pigments or any mixtures thereofThe jacketing layer comprises preferably a pigment or carbon black, orboth, in conventionally used amounts.

The jacketing composition of the cable of the invention comprisestypically at least 50 wt %, preferably at least 60 wt %, more preferablyat least 70 wt % to 100 wt %, of the jacketing polyolefin based on thetotal weight of the polymer component(s) present in the jacketingcomposition. However, it is to be understood herein that the jacketingcomposition may comprise further component(s) other than polymercomponents, such as additive(s) which may optionally be added in amixture with a carrier polymer, i.e. in so called master batch.

AC Power Cable of the Invention

An alternating current (AC) power cable of the invention is verysuitable for AC power cables, especially for power cables operating atvoltages between 6 kV and 36 kV (medium voltage (MV) cables) and atvoltages higher than 36 kV, known as high voltage (HV) cables and extrahigh voltage (EHV) cables, which EHV cables operate, as well known, atvery high voltages. The terms have well known meanings and indicate theoperating level of such cables. The most preferred AC power cable is a

Accordingly, the polymer composition with advantageous low dielectriclosses properties is highly suitable HV or EHV AC power cable whichoperates at voltages higher than 36 kV, preferably at voltages of 40 kVor higher, even at voltages of 50 kV or higher. EHV AC power cablesoperate at very high voltage ranges e.g as high as up to 800 kV, howeverwithout limiting thereto.

The invention also provides a process for producing an alternatingcurrent (AC) power cable, preferably a HV or EHV AC power cable, asdefined above or claims, wherein the process comprises the steps of

-   -   applying on a conductor an inner semiconductive layer comprising        a first semiconductive composition, an insulation layer        comprising a polymer composition, an outer semiconductive layer        comprising a second semiconductive composition, and optionally,        and preferably, a jacketing layer comprising a jacketing        composition, and    -   optionally, and preferably, crosslinking at least the polyolefin        of the polymer composition of the insulation layer, optionally,        and preferably, the first semiconductive composition of the        inner semiconductive layer, optionally the second semiconductive        composition of the outer semiconductive layer and optionally the        jacketing composition of the optional jacketing layer, in the        presence of a crosslinking agent and at crosslinking conditions.

In the preferred embodiment of the HV or EHV AC power cable productionprocess of the invention the process comprises the steps of

(a)

-   -   providing and mixing, preferably meltmixing in an extruder, an        optionally crosslinkable first semiconductive composition        comprising a polyolefin, a conductive filler, preferably carbon        black, and optionally further component(s) for the inner        semiconductive layer,    -   providing and mixing, preferably meltmixing in an extruder, a        crosslinkable polymer composition of the invention for the        insulation layer,    -   providing and mixing, preferably meltmixing in an extruder, an        optionally crosslinkable second semiconductive composition which        comprises a polyolefin, a conductive filler, preferably carbon        black, and optionally further component(s) for the outer        semiconductive layer,    -   providing and mixing, preferably meltmixing in an extruder, an        optionally crosslinkable jacketing composition which comprises a        polyolefin and optionally further component(s) for the outer        semiconductive layer,

(b) applying on a conductor, preferably by coextrusion,

-   -   a meltmix of the first semiconductive composition obtained from        step (a) to form the inner semiconductive layer,    -   a meltmix of polymer composition of the invention obtained from        step (a) to form the insulation layer,    -   a meltmix of the second semiconductive composition obtained from        step (a) to form the outer semiconductive layer,    -   a meltmix of the jacketing composition obtained from step (a) to        form the shield jacketing layer, and

(c) optionally crosslinking at crosslinking conditions one or more ofthe polymer composition of the insulation layer, the semiconductivecomposition of the inner semiconductive layer, the semiconductivecomposition of the outer semiconductive layer, and the jacketingcomposition of the jacketing layer of the obtained cable, preferably atleast the polymer composition of the insulation layer, more preferablythe polymer composition of the insulation layer, at least thesemiconductive composition of the inner semiconductive layer, optionallythe semiconductive composition of the outer semiconductive layer andoptionally the jacketing composition of the jacketing layer.

Melt mixing means mixing above the melting point of at least the majorpolymer component(s) of the obtained mixture and is typically carriedout in a temperature of at least 10-15° C. above the melting orsoftening point of polymer component(s).

The term “(co)extrusion” means herein that in case of two or morelayers, said layers can be extruded in separate steps, or at least twoor all of said layers can be coextruded in a same extrusion step, aswell known in the art. The term “(co)extrusion” means herein also thatall or part of the layer(s) are formed simultaneously using one or moreextrusion heads. For instance triple extrusion can be used for formingthree cable layers.

As well known, the polymer composition of the invention and the firstand second semiconductive compositions and the optional, and preferable,jacketing composition can be produced before or during the cableproduction process. Moreover the polymer composition of the insulationlayer, the first and second semiconductive compositions and theoptional, and preferable, jacketing composition can each independentlycomprise part or all of the component(s) thereof before introducing tothe (melt)mixing step a) of the cable production process.

Preferably, said part or all of the polymer composition, preferably atleast the polyolefin, is in form of powder, grain or pellets, whenprovided to the cable production process. Pellets can be of any size andshape and can be produced by any conventional pelletizing device, suchas a pelletizing extruder.

According to one embodiment, at least the polymer composition comprisessaid optional further component(s). In this embodiment part or all ofsaid further component(s) may e.g. be added

1) by meltmixing to the polyolefin, which may be in a form as obtainedfrom a polymerization process, and then the obtained meltmix ispelletized, and/or

2) by mixing to the pellets of the polyolefin which pellets may alreadycontain part of said further component(s). In this option 2) part or allof the further component(s) can be meltmixed together with the pelletsand then the obtained meltmix is pelletized; and/or part or all of thefurther components can be impregnated to the solid pellets.

In an alternative second embodiment, the polymer composition may beprepared in connection with the cable production line e.g. by providingthe polyolefin, preferably in form of pellets which may optionallycomprise part of the further component(s), and combined with all or restof the further component(s) in the mixing step a) to provide a (melt)mixfor the step b) of the process of the invention. In case the pellets ofthe polyolefin contain part of the further component(s), then thepellets may be prepared as described in the above first embodiment.

The further component(s) is preferably selected at least from one ormore additive(s), preferably at least from free radical generatingagent(s), more preferably from peroxide(s), optionally, and preferably,from antioxidant(s) and optionally from scorch retardant(s) as mentionedabove.

The mixing step a) of the provided polymer composition, the first andsecond semiconductive compositions and the optional, and preferable,jacketing composition is preferably carried out in the cable extruder.The step a) may optionally comprise a separate mixing step, e.g. in amixer, preceding the cable extruder. Mixing in the preceding separatemixer can be carried out by mixing with or without external heating(heating with an external source) of the component(s). Any furthercomponent(s) of the polymer composition or the first and secondsemiconductive composition and the optional, and preferable, jacketingcomposition, if present and added during the cable production process,can be added at any stage and any point(s) in to the cable extruder, orto the optional separate mixer preceding the cable extruder. Theaddition of additives can be made simultaneously or separately as such,preferably in liquid form, or in a well known master batch, and at anystage during the mixing step (a).

It is preferred that the (melt)mix of the polymer composition obtainedfrom (melt)mixing step (a) consists of the polyolefin of the inventionas the sole polymer component. The optional, and preferable, additive(s)can be added to polymer composition as such or as a mixture with acarrier polymer, i.e. in a form of so-called master batch.

Most preferably the mixture of the polymer composition of the insulationlayer and the mixture of each of the first and second semiconductivecompositions and the optional, and preferable, jacketing compositionobtained from step (a) is a meltmix produced at least in an extruder.

In the preferred embodiment at least the polymer composition of theinsulation layer of the invention is provided to the cable productionprocess in a form of premade pellets.

In a preferred embodiment of the cable production process, a crosslinkedMV, HV or EHV AC power cable, more preferably a crosslinked HV or EHV ACpower cable, is produced, which comprises a conductor surrounded by aninner semiconductive layer comprising, preferably consisting of, a firstsemiconductive composition, an insulation layer comprising, preferablyconsisting of, a crosslinkable polymer composition of the inventioncomprising a polyolefin and a crosslinking agent, preferably peroxide,as defined above, an outer semiconductive layer comprising, preferablyconsisting of, a second semiconductive composition, and the optional,and preferable , jacketing layer comprising, preferably consisting of,the jacketing composition, wherein at least the polymer composition ofthe insulation layer is crosslinked in the presence of said crosslinkingagent, more preferably, wherein at least the first semiconductivecomposition of the inner semiconductive layer and the polymercomposition of the insulation layer are crosslinked.

If crosslinked, then the crosslinking agent(s) can already be present inthe first and second semiconductive composition before introducing tothe crosslinking step (c) or introduced during the crosslinking step(c). Peroxide is the preferred crosslinking agent for said first andsecond semiconductive compositions and for the optional, and preferable,jacketing composition in case any of said layer(s) are crosslinked, andis then preferably included to the pellets of semiconductivecompositions and the pellets of the jacketing composition before thecomposition is used in the cable production process as described above.

Crosslinking can be carried out at increased temperature which ischosen, as well known, depending on the type of crosslinking agent. Forinstance temperatures above 150° C. are typical, however withoutlimiting thereto.

Insulating layers for MV, HV or EHV, preferably for HV or EHV, AC powercables generally have a thickness of at least 2 mm, typically of atleast 2.3 mm, when measured from a cross section of the insulation layerof the cable, and the thickness increases with increasing voltage thecable is designed for.

Determination Methods

Unless otherwise stated in the description or experimental part thefollowing methods were used for the property determinations.

Wt %: % by weight

Melt Flow Rate

The melt flow rate (MFR) is determined according to ISO 1133 and isindicated in g/10 min. The MFR is an indication of the flowability, andhence the processability, of the polymer. The higher the melt flow rate,the lower the viscosity of the polymer. The MFR is determined at 190° C.for polyethylenes and may be determined at different loadings such as2.16 kg (MFR₂) or 21.6 kg (MFR₂₁).

Density

The density was measured according to ISO 1183-2. The sample preparationwas executed according to ISO 1872-2 Table 3 Q (compression moulding).

Molecular Weight

The Mz, Mw, Mn, and MWD are measured by Gel Permeation Chromatography(GPC) for low molecular weight polymers as known in the field.

Comonomer Contents

a) Quantification of Alpha-Olefin Content in Linear Low DensityPolyethylenes and Low Density Polyethylenes by NMR Spectroscopy:

The comonomer content was determined by quantitative 13C nuclearmagnetic resonance (NMR) spectroscopy after basic assignment (J. RandallJMS—Rev. Macromol. Chem. Phys., C29(2&3), 201-317 (1989)). Experimentalparameters were adjusted to ensure measurement of quantitative spectrafor this specific task.

Specifically solution-state NMR spectroscopy was employed using a BrukerAvanceIII 400 spectrometer. Homogeneous samples were prepared bydissolving approximately 0.200 g of polymer in 2.5 ml ofdeuterated-tetrachloroethene in 10 mm sample tubes utilizing a heatblock and rotating tube oven at 140° C. Proton decoupled 13C singlepulse NMR spectra with NOE (powergated) were recorded using thefollowing acquisition parameters: a flip-angle of 90 degrees, 4 dummyscans, 4096 transients an acquisition time of 1.6 s, a spectral width of20 kHz, a temperature of 125° C., a bilevel WALTZ proton decouplingscheme and a relaxation delay of 3.0 s. The resulting FID was processedusing the following processing parameters: zero-filling to 32 k datapoints and apodisation using a gaussian window function; automaticzeroth and first order phase correction and automatic baselinecorrection using a fifth order polynomial restricted to the region ofinterest.

Quantities were calculated using simple corrected ratios of the signalintegrals of representative sites based upon methods well known in theart.

b) Comonomer Content of Polar Comonomers in Low Density Polyethylene

(1) Polymers Containing >6 wt. % Polar Comonomer Units

Comonomer content (wt %) was determined in a known manner based onFourier transform infrared spectroscopy (FTIR) determination calibratedwith quantitative nuclear magnetic resonance (NMR) spectroscopy. Belowis exemplified the determination of the polar comonomer content ofethylene ethyl acrylate, ethylene butyl acrylate and ethylene methylacrylate. Film samples of the polymers were prepared for the FTIRmeasurement: 0.5-0.7 mm thickness was used for ethylene butyl acrylateand ethylene ethyl acrylate and 0.10 mm film thickness for ethylenemethyl acrylate in amount of >6 wt %. Films were pressed using a Specacfilm press at 150° C., approximately at 5 tons, 1-2 minutes, and thencooled with cold water in a not controlled manner. The accuratethickness of the obtained film samples was measured.

After the analysis with FTIR, base lines in absorbance mode were drawnfor the peaks to be analysed. The absorbance peak for the comonomer wasnormalised with the absorbance peak of polyethylene (e.g. the peakheight for butyl acrylate or ethyl acrylate at 3450 cm⁻¹ was dividedwith the peak height of polyethylene at 2020 cm⁻¹). The NMR spectroscopycalibration procedure was undertaken in the conventional manner which iswell documented in the literature, explained below.

For the determination of the content of methyl acrylate a 0.10 mm thickfilm sample was prepared. After the analysis the maximum absorbance forthe peak for the methylacrylate at 3455 cm⁻¹ was subtracted with theabsorbance value for the base line at 2475 cm⁻¹(A_(methylacrylate)−A₂₄₇₅). Then the maximum absorbance peak for thepolyethylene peak at 2660 cm⁻¹ was subtracted with the absorbance valuefor the base line at 2475 cm⁻¹ (A₂₆₆₀−A₂₄₇₅). The ratio between(A_(methylacrylate)−A₂₄₇₅) and (A₂₆₆₀−A₂₄₇₅) was then calculated in theconventional manner which is well documented in the literature.

The weight-% can be converted to mol-% by calculation. It is welldocumented in the literature.

Quantification of Copolymer Content in Polymers by NMR Spectroscopy

The comonomer content was determined by quantitative nuclear magneticresonance (NMR) spectroscopy after basic assignment (e.g. “NMR Spectraof Polymers and Polymer Additives”, A. J. Brandolini and D. D. Hills,2000, Marcel Dekker, Inc. New York). Experimental parameters wereadjusted to ensure measurement of quantitative spectra for this specifictask (e.g “200 and More NMR Experiments: A Practical Course”, S. Bergerand S. Braun, 2004, Wiley-VCH, Weinheim). Quantities were calculatedusing simple corrected ratios of the signal integrals of representativesites in a manner known in the art.

(2) Polymers Containing 6 wt. % or Less Polar Comonomer Units

Comonomer content (wt. %) was determined in a known manner based onFourier transform infrared spectroscopy (FTIR) determination calibratedwith quantitative nuclear magnetic resonance (NMR) spectroscopy. Belowis exemplified the determination of the polar comonomer content ofethylene butyl acrylate and ethylene methyl acrylate. For the FT-IRmeasurement a film samples of 0.05 to 0.12 mm thickness were prepared asdescribed above under method 1). The accurate thickness of the obtainedfilm samples was measured.

After the analysis with FT-IR base lines in absorbance mode were drawnfor the peaks to be analysed. The maximum absorbance for the peak forthe comonomer (e.g. for methylacrylate at 1164 cm⁻¹ and butylacrylate at1165 cm⁻¹) was subtracted with the absorbance value for the base line at1850 cm⁻¹ (A_(polar comonomer)−A₁₈₅₀). Then the maximum absorbance peakfor polyethylene peak at 2660 cm⁻¹ was subtracted with the absorbancevalue for the base line at 1850 cm⁻¹ (A₂₆₆₀−A₁₈₅₀). The ratio between(A_(comonomer)−A₁₈₅₀) and (A₂₆₆₀−A₁₈₅₀) was then calculated. The NMRspectroscopy calibration procedure was undertaken in the conventionalmanner which is well documented in the literature, as described aboveunder method 1).

The weight-% can be converted to mol-% by calculation. It is welldocumented in the literature.

Test for Tan δ Measurements on 10 kV Cables

Cable Production

Polymers pellets of the test polymer composition were used to producethe insulation layer of the test 10 kV cables on a Maillefer pilot cableline of CCV type. The cables have 3.4 mm nominal insulation thickness(the inner semiconductive layer is 0.9 mm thick and the outersemiconductive layer is 1 mm thick). The conductor cross section was 50mm² stranded aluminum. The cable was produced as a 1+2 construction(e.g. first the inner semiconductive layer was applied onto theconductor and then the remaining two layer were applied via the sameextrusion head to the conductor having already the inner semiconductivelayer applied). The semiconductive material used as inner and outsemiconductive material was LE0592 (a commercially semiconductivematerial supplied by Borealis). The cable cores were produced with aline speed of 1.6 m/min.

Cable Length:

Preparation of Cable Sample:

12.5 m of each cable were available for the tests; active test length inthe loss factor tests was approximately 11 m. The length is chosen to bein accordance with IEC 60502-2; i.e. ≥10 m active test length betweenthe guard rings of the test object.

Conditioning:

The cables are thermally treated in a ventilated oven at 70° C. for 72hours before the measurements. The samples are afterwards kept in sealedaluminum bags until the tan δ measurements are done.

Test Method:

Both ends of the loss factor cables were equipped with electric fieldgrading cloths. Each termination was 0.7 m long. The ends were put intoplastic bags that were filled with SF₆-gas and sealed by tapes. TheSF₆-gas was used to increase the corona inception voltage beyond themaximum test voltage of ˜55 kV.

20 cm from the stress cones guard rings were introduced. A 2 mm gap wasopened in the insulation screen. A 5 cm long thick walled heat shrinktube (Raychem) was used over the guard rings to avoid any influence ofpartial discharges and/or leakage currents from the highly stressedterminations during the measurements.

The active test length was wrapped in a 0.45 m wide and 0.2 mm thickAl-foil (6-7 layers). Afterwards this was covered with a continuousinsulating heat shrinkable tube.

All tan δ-measurements were performed with the cable coiled inside alarge ventilated oven. The terminations were mounted and connected tothe high voltage transformer outside the ventilated oven. The guardrings were also located outside of the oven.

In order to reach isothermal conditions within the entire cable a periodof 2 hours was required between the measurements on each temperaturelevel. The cable is thus heated by this oven, and not by conductorheating.

The 50 Hz test voltages corresponding to 5, 10, 15, 20 and 25 kV/mmconductor stress were determined after the dimensions of the cables weremeasured.

The tan δ bridge was of the type Schering Bridge Tettex 2801 H1-64. Thesystem was checked prior to the measurements by the use of tan δstandards.

Method for Determination of the Amount of Double Bonds in the PolymerComposition or in the Polymer

The method describes generally the determination of different type ofdouble bonds and the part of the description is used which describes thedetermination of the vinyl group content.

A) Quantification of the Amount of Carbon-Carbon Double Bonds by IRSpectroscopy

Quantitative infrared (IR) spectroscopy was used to quantify the amountof carbon-carbon doubles (C═C). Calibration was achieved by priordetermination of the molar extinction coefficient of the C═C functionalgroups in representative low molecular weight model compounds of knownstructure.

The amount of each of these groups (N) was determined as number ofcarbon-carbon double bonds per thousand total carbon atoms (C═C/1000C)via:

N=(A×14)/(E×L×D)

were A is the maximum absorbance defined as peak height, E the molarextinction coefficient of the group in question (l·mol⁻¹·mm⁻¹), L thefilm thickness (mm) and D the density of the material (g·cm⁻¹).

The total amount of C═C bonds per thousand total carbon atoms can becalculated through summation of N for the individual C═C containingcomponents.

For polyethylene samples solid-state infrared spectra were recordedusing a FTIR spectrometer (Perkin Elmer 2000) on compression mouldedthin (0.5-1.0 mm) films at a resolution of 4 cm⁻¹ and analysed inabsorption mode.

1) Polymer Compositions Comprising Polyethylene Homopolymers andCopolymers, Except Polyethylene Copolymers with >0.4 wt % PolarComonomer

For polyethylenes three types of C═C containing functional groups werequantified, each with a characteristic absorption and each calibrated toa different model compound resulting in individual extinctioncoefficients:

-   -   vinyl (R—CH═CH2) via 910 cm⁻¹ based on 1-decene [dec-1-ene]        giving E=13.13 l·mol⁻¹·mm⁻¹    -   vinylidene (RR′C═CH2) via 888 cm⁻¹ based on 2-methyl-1-heptene        [2-methyhept-1-ene] giving E=18.24 l·mol⁻¹·mm⁻¹    -   trans-vinylene (R—CH═CH—R′) via 965 cm⁻¹ based on trans-4-decene        [(E)-dec-4-ene] giving E=15.14 l·mol⁻¹·mm⁻¹

For polyethylene homopolymers or copolymers with <0.4 wt % of polarcomonomer linear baseline correction was applied between approximately980 and 840 cm⁻¹.

2) Polymer Compositions Comprising Polyethylene Copolymers with >0.4 wt% Polar Comonomer

For polyethylene copolymers with >0.4 wt % of polar comonomer two typesof C═C containing functional groups were quantified, each with acharacteristic absorption and each calibrated to a different modelcompound resulting in individual extinction coefficients:

-   -   vinyl (R—CH═CH2) via 910 cm⁻¹ based on 1-decene [dec-1-ene]        giving E=13.13 l·mol⁻¹·mm⁻¹    -   vinylidene (RR′C═CH2) via 888 cm⁻¹ based on 2-methyl-1-heptene        [2-methyhept-1-ene] giving E=18.24 l·mol⁻¹·mm⁻¹

EBA:

For poly(ethylene-co-butylacrylate) (EBA) systems linear baselinecorrection was applied between approximately 920 and 870 cm⁻¹.

EMA:

For poly(ethylene-co-methylacrylate) (EMA) systems linear baselinecorrection was applied between approximately 930 and 870 cm⁻¹.

3) Polymer Compositions Comprising Unsaturated Low Molecular WeightMolecules

For systems containing low molecular weight C═C containing speciesdirect calibration using the molar extinction coefficient of the C═Cabsorption in the low molecular weight species itself was undertaken.

B) Quantification of Molar Extinction Coefficients by IR Spectroscopy

The molar extinction coefficients were determined according to theprocedure given in ASTM D3124-98 and ASTM D6248-98. Solution-stateinfrared spectra were recorded using a FTIR spectrometer (Perkin Elmer2000) equipped with a 0.1 mm path length liquid cell at a resolution of4 cm⁻¹.

The molar extinction coefficient (E) was determined as l·mol⁻¹·mm⁻¹ via:

E=A/(C×L)

were A is the maximum absorbance defined as peak height, C theconcentration (mol·l⁻¹) and L the cell thickness (mm).

At least three 0.18 mol·l⁻¹ solutions in carbondisulphide (CS₂) wereused and the mean value of the molar extinction coefficient determined.

Experimental Part

Preparation of Polyolefins of the Examples of the Present Invention andthe Reference Examples

The polyolefins were low density polyethylenes produced in a highpressure reactor. The production of inventive and reference polymers isdescribed below. As to CTA feeds, e.g. the PA content can be given asliter/hour or kg/h and converted to either units using a density of PAof 0.807 kg/liter for the recalculation.

COMPARATIVE EXAMPLE 1 Polyethylene Polymer Produced in a High PressureReactor

Purified ethylene was liquefied by compression and cooling to a pressureof 90 bars and a temperature of −30° C. and split up into to two equalstreams of roughly 14 tons/hour each. The CTA (methyl ethyl ketone(MEK)), air and a commercial peroxide radical initiator dissolved in asolvent were added to the two liquid ethylene streams in individualamounts. The two mixtures were separately pumped through an array of 4intensifiers to reach pressures of 2100-2300 bars and exit temperaturesof around 40° C. These two streams were respectively fed to the front(zone 1) (50%) and side (zone 2) (50%) of a split-feed two-zone tubularreactor. The inner diameters and lengths of the two reactor zones were32 mm and 200 m for zone 1 and 38 mm and 400 m for zone 2. MEK was addedin amounts of around 216 kg/h to the front stream to maintain a MFR2 ofaround 2 g/10 min. The front feed stream was passed through a heatingsection to reach a temperature sufficient for the exothermalpolymerization reaction to start. The reaction reached peak temperatureswere around 250° C. and around 318° C. in the first and second zones,respectively. The side feed stream cooled the reaction to an initiationtemperature of the second zone of 165-170° C. Air and peroxide solutionwas added to the two streams in enough amounts to reach the target peaktemperatures. The reaction mixture was depressurized by product valve,cooled and polymer was separated from unreacted gas.

COMPARATIVE EXAMPLE 2 Polyethylene Polymer Produced in a High PressureReactor

Purified ethylene was liquefied by compression and cooling to a pressureof 90 bars and a temperature of −30° C. and split up into to two equalstreams of roughly 14 tons/hour each. The CTA (methyl ethyl ketone,MEK), air and a commercial peroxide radical initiator dissolved in asolvent were added to the two liquid ethylene streams in individualamounts. Here also 1,7-octadiene was added to the reactor in amount ofaround 24 kg/h. The two mixtures were separately pumped through an arrayof 4 intensifiers to reach pressures of 2200-2300 bars and exittemperatures of around 40° C. These two streams were respectively fed tothe front (zone 1) (50%) and side (zone 2) (50%) of a split-feedtwo-zone tubular reactor. The inner diameters and lengths of the tworeactor zones were 32 mm and 200 m for zone 1 and 38 mm and 400 m forzone 2. MEK was added in amounts of around 205 kg/h to the front streamto maintain a MFR2 of around 2 g/10 min. The front feed stream waspassed through a heating section to reach a temperature sufficient forthe exothermal polymerization reaction to start. The reaction reachedpeak temperatures were around 253° C. and around 290° C. in the firstand second zones, respectively. The side feed stream cooled the reactionto an initiation temperature of the second zone of around 168° C. Airand peroxide solution was added to the two streams in enough amounts toreach the target peak temperatures. The reaction mixture wasdepressurized by product valve, cooled and polymer was separated fromunreacted gas.

INVENTIVE EXAMPLE 1 Polyethylene Polymer Produced in a High PressureReactor

Ethylene with recycled CTA was compressed in a 5-stage precompressor anda 2-stage hyper compressor with intermediate cooling to reach initialreaction pressure of ca 2700 bar. The total compressor throughput was ca30 tons/hour. In the compressor area approximately 5.6 kg/hour ofpropion aldehyde was added together with approximately 89 kgpropylene/hour as chain transfer agents to maintain an MFR of 1.9 g/10min. The compressed mixture was heated to 163° C. in a preheatingsection of a front feed three-zone tubular reactor with an innerdiameter of ca 40 mm and a total length of 1200 meters. A mixture ofcommercially available peroxide radical initiators dissolved inisododecane was injected just after the preheater in an amountsufficient for the exothermal polymerization reaction to reach peaktemperatures of ca 286° C. after which it was cooled to approximately215° C. The subsequent 2^(nd) and 3^(rd) peak reaction temperatures were285° C. and 268° C. respectively with a cooling in between to 230° C.The reaction mixture was depressurised by a kick valve, cooled andpolymer was separated from unreacted gas.

INVENTIVE EXAMPLE 2 Polyethylene Polymer Produced in a High PressureReactor

Ethylene with recycled CTA was compressed in a 5-stage precompressor anda 2-stage hyper compressor with intermediate cooling to reach initialreaction pressure of ca 2580 bar. The total compressor throughput was ca30 tons/hour. In the compressor area approximately 14.4 kg/hour ofpropion aldehyde was added to maintain an MFR of around 2.0 g/10 min.The compressed mixture was heated to 164° C. in a preheating section ofa front feed three-zone tubular reactor with an inner diameter of ca 40mm and a total length of 1200 meters. A mixture of commerciallyavailable peroxide radical initiators dissolved in isododecane wasinjected just after the preheater in an amount sufficient for theexothermal polymerization reaction to reach peak temperatures of ca 305°C. after which it was cooled to approximately 208° C. The subsequent2^(nd) and 3^(rd) peak reaction temperatures were 286° C. and 278° C.respectively with a cooling in between to 237° C. The reaction mixturewas depressurised by a kick valve, cooled and polymer was separated fromunreacted gas.

INVENTIVE EXAMPLE 3 Polyethylene Polymer Produced in a High PressureReactor

Ethylene with recycled CTA was compressed in a 5-stage precompressor anda 2-stage hyper compressor with intermediate cooling to reach initialreaction pressure of ca 2800 bar. The total compressor throughput was ca30 tons/hour. In the compressor area approximately 3.6 kg /hour ofpropion aldehyde was added together with approximately 138 kg/hour ofpropylene as chain transfer agents to maintain an MFR of 2.1 g/10 min.Here also 1,7-octadiene was added to the reactor in amount of 30.7 kg/h.The compressed mixture was heated to 167° C. in a preheating section ofa front feed three-zone tubular reactor with an inner diameter of ca 40mm and a total length of 1200 meters. A mixture of commerciallyavailable peroxide radical initiators dissolved in isododecane wasinjected just after the preheater in an amount sufficient for theexothermal polymerization reaction to reach peak temperatures of ca 271°C. after which it was cooled to approximately 195° C. The subsequent2^(nd) and 3^(rd) peak reaction temperatures were 269° C. and 247° C.respectively with a cooling in between to 216° C. The reaction mixturewas depressurised by a kick valve, cooled and polymer was separated fromunreacted gas.

INVENTIVE EXAMPLE 4 Polyethylene Polymer Produced in a High PressureReactor

Ethylene with recycled CTA was compressed in a 5-stage precompressor anda 2-stage hyper compressor with intermediate cooling to reach initialreaction pressure of ca 2745 bar. The total compressor throughput was ca30 tons/hour. In the compressor area approximately 2.8 kg /hour ofpropion aldehyde was added together with approximately 77 kg/hour ofpropylene as chain transfer agents to maintain an MFR of 1.8 g/10 min.Here also 1,7-octadiene was added to the reactor in amount of 28.7 kg/hand butyl acrylate to an amount of around 26 kg/h. The compressedmixture was heated to 161° C. in a preheating section of a front feedthree-zone tubular reactor with an inner diameter of ca 40 mm and atotal length of 1200 meters. A mixture of commercially availableperoxide radical initiators dissolved in isododecane was injected justafter the preheater in an amount sufficient for the exothermalpolymerization reaction to reach peak temperatures of ca 286° C. afterwhich it was cooled to approximately 233° C. The subsequent 2^(nd) and3^(rd) peak reaction temperatures were 280° C. and 266° C. respectivelywith a cooling in between to 237° C. The reaction mixture wasdepressurised by a kick valve, cooled and polymer was separated fromunreacted gas.

Experimental Results:

Mineral oil=mineral oil based lubricant, M-RARUS PE KPL 201, supplierExxonMobil

PAG=polyalkylkene glycol based lubricant, Syntheso D201N from Klueber.

PA=propion aldehyde (CAS number: 123-38-6)

MEK=methyl ethyl ketone.

Base Resin Inv. Inv. Inv. Inv. Ref. Ref. Properties ex. 1 ex. 2 ex. 3ex. 4 ex. 1 ex. 2 MFR 2.16 kg, 1.9 2.1 2.1 1.8 2.0 2.0 190° C. [g/10min] Density 924 922 921.3 921.2 922 922 [kg/m³] Vinyl [C═C/ 0.32 0.130.59 0.53 0.11 0.25 1000 C]

The results in below tables show that the inventive polymer compositionsproduced in HP process using mineral oil as the compressor lubricanthave reduced the losses at high stress and high temperature expressed astan delta measured at 50 Hz.

TABLE 1 Tan delta (50 Hz) at 25 kV/mm and 130° C. of crosslinked 10 kVcables. Polymer Compressor oil Tan delta (10⁻⁴) Comp Ex 1 PAG 13.2 InvEx 1 Mineral oil 5.4 Inv Ex 2 Mineral oil 6.8

TABLE 2 Tan delta (50 Hz) at 25 kV/mm and 130° C. of crosslinked 10 kVcables. Polymer Compressor oil Tan delta (10⁻⁴) Comp Ex 2 PAG 16.1 InvEx 3 Mineral oil 3.4 Inv Ex 4 Mineral oil 4.8

The jacketing layer is coextruded in a cable extruder in a conventionalmanner as the shield (outer) layer of the final cable of the invention.The preferable jacketing layer of the cable further contributes to theimproved AC electrical properties by means of providing a mechanicallyprotective layer.

What is claimed is:
 1. An alternating current (AC) power cable,comprising a conductor surrounded by at least an inner semiconductivelayer comprising a first semiconductive composition, an insulation layercomprising a polymer composition, an outer semiconductive layercomprising a second semiconductive composition and optionally ajacketing layer comprising a jacketing composition, in that order,wherein the polymer composition of the insulation layer comprises apolyolefin and a crosslinking agent, wherein the polyolefin is anunsaturated low density polyethylene (LDPE) copolymer of ethylene withone or more polyunsaturated comonomers, and wherein the polymercomposition of the insulation layer has a dielectric loss expressed astan δ (50 Hz) of 12.0×10⁻⁴ or less, when measured at 25 kV/mm and 130°C. according to “Test for Tan δ measurements on 10 kV cables” asdescribed in the description part under “Determination methods”; andwherein the polyolefin is obtainable by a high pressure processcomprising: (a) compressing one or more monomer(s) under pressure in acompressor, using a compressor lubricant comprising a mineral oil forlubrication, (b) polymerizing a monomer optionally together with one ormore comonomer(s) in a polymerization zone, and (c) separating theobtained polyolefin from the unreacted products and recovering theseparated polyolefin in a recovery zone.
 2. The cable according to claim1, wherein the polymer composition of the insulation layer has adielectric loss expressed as tan δ (50 Hz) of 11.0×10⁻⁴ or less, whenmeasured at 25 kV/mm and 130° C. according to “Test for Tan δmeasurements on 10 kV cables” as described in the description part under“Determination methods”.
 3. The cable according to claim 1, wherein thepolymer composition of the insulation layer has a dielectric lossexpressed as tan δ (50 Hz) of 0.01×10⁻⁴-10.0×10⁻⁴, when measured at 25kV/mm and 130° C. according to “Test for Tan δ measurements on 10 kVcables” as described in the description part under “Determinationmethods”.
 4. The cable according to claim 1, wherein the compressorlubricant comprises a white oil as the mineral oil and is suitable forproduction of polymers for food or medical industry.
 5. The cableaccording to claim 1, wherein the mineral oil is a white mineral oilwhich meets requirements given for white mineral oil in EuropeanDirective 2002/72/EC of 6 Aug. 2002, Annex V, for plastics used in foodcontact.
 6. The cable according to claim 1, wherein the crosslinkingagent is peroxide.
 7. The cable according to claim 1, wherein each ofthe inner and outer semiconductive composition comprises independently aconductive filler.
 8. The cable according to claim 1, wherein the cablecomprises a jacketing layer comprising a jacketing composition.
 9. Thecable according to claim 1, wherein at least the polymer composition ofthe insulation layer is crosslinked in the presence of said crosslinkingagent.
 10. The cable according to claim 1, wherein at least the firstsemiconductive composition of the inner semiconductive layer and thepolymer composition of the insulation layer are crosslinked.
 11. Thecable according to claim 1, which is a medium voltage (MV) cable, a highvoltage (HV) cable, or an extra high voltage (EHV) cable.
 12. The cableaccording to claim 1, wherein the unsaturated LDPE copolymer of ethylenewith one or more polyunsaturated comonomers comprises one or more othercomonomer(s).
 13. The cable according to claim 1, wherein thepolyunsaturated comonomer consists of a straight carbon chain with atleast 8 carbon atoms and at least 4 carbons between non-conjugateddouble bonds, of which at least one is terminal.
 14. The cable accordingto claim 13, wherein said polyunsaturated comonomer is a diene whichcomprises at least eight carbon atoms, the first carbon-carbon doublebond being terminal and the second carbon-carbon double bond beingnon-conjugated to the first carbon-carbon double bond.
 15. The cableaccording to claim 14, wherein said diene is selected from C8- toC14-non-conjugated diene or mixtures thereof.
 16. The cable according toclaim 15, wherein said diene is selected from the group consisting of1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene,7-methyl-1,6-octadiene, 9-methyl-1,8-decadiene, and mixtures thereof.17. An alternating current (AC) power cable, comprising a conductorsurrounded by at least an optionally crosslinked inner semiconductivelayer comprising an optionally crosslinked first semiconductivecomposition, a crosslinked insulation layer comprising a crosslinkedpolymer composition, an optionally crosslinked outer semiconductivelayer comprising an optionally crosslinked second semiconductivecomposition and optionally a jacketing layer comprising a jacketingcomposition, in that order, wherein the crosslinked polymer compositionof the insulation layer comprises a crosslinked polyolefin wherein thepolyolefin is an unsaturated low density polyethylene (LDPE) copolymerof ethylene with one or more polyunsaturated comonomers, and wherein thecrosslinked polymer composition of the insulation layer has a dielectricloss expressed as tan δ (50 Hz) of 12.0×10⁻⁴ or less, when measured at25 kV/mm and 130° C. according to “Test for Tan δ measurements on 10 kVcables” as described in the description part under “Determinationmethods”; and wherein the polyolefin is obtainable by a high pressureprocess comprising: (a) compressing one or more monomer(s) underpressure in a compressor, using a compressor lubricant comprising amineral oil for lubrication, (b) polymerizing a monomer optionallytogether with one or more comonomer(s) in a polymerization zone, and (c)separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone.
 18. A processfor producing an alternating current (AC) power cable wherein theprocess comprises: applying on a conductor an inner semiconductive layercomprising a first semiconductive composition, an insulation layercomprising a polymer composition, an outer semiconductive layercomprising a second semiconductive composition, and optionally ajacketing layer comprising a jacketing composition; wherein the polymercomposition of the insulation layer comprises a polyolefin and acrosslinking agent, wherein the polyolefin is an unsaturated low densitypolyethylene (LDPE) copolymer of ethylene with one or morepolyunsaturated comonomers, and wherein the polyolefin is obtainable bya high pressure process comprising: (a) compressing one or moremonomer(s) under pressure in a compressor, using a compressor lubricantcomprising a mineral oil for lubrication, (b) polymerizing a monomeroptionally together with one or more comonomer(s) in a polymerizationzone, and (c) separating the obtained polyolefin from the unreactedproducts and recovering the separated polyolefin in a recovery zone;crosslinking at least said insulation layer so as to form a crosslinkedpolymer composition of the insulation layer which has a dielectric lossexpressed as tan δ (50 Hz) of 12.0×10⁻⁴ or less, when measured at 25kV/mm and 130° C. according to “Test for Tan δ measurements on 10 kVcables” as described in the description part under “Determinationmethods”.